This icon commission by TwilightSaint is one of the best icon designs for dragons that I have seen. It's very vibrant and detailed, which is something that I greatly appreciate when seeing this type of artwork.
The shading and highlights, with the combination of color shades and highlighted detail lines, is nothing short of beautiful. It is also spot-on with the source of light.
Despite the cool colors of the dragon itself, the background and the dragon seem to work together due to the darkened and slightly dulled color of the skin. (Though highlights brighten this to bring out the contours and to give the dragon its realistic touch.) Through it all, there is no conflict between the cool blue colors and the bright red background.
What I will have to say that does not come up to par is the background and the spot of brownish-red color behind the dragon. Underneath the chin is a streak of dark red that does not seem to be consistent with the rest of the background beside and above it; above the head are rusty spots of red with a far different texture to them.
For behind the dragon's neck, the brown spot does not fit in hardly at all; it does not have a design that suits the background or the dragon, depending on which part it adorns. It also has sketch marks in black at the base, which seem to stick out a little much.
The background and that tidbit were the only things that I found could use improvement within the icon. I hope this helped!